Instrumentation for real-time Biomolecular Interaction Analysis (BIA) is commercially available from Biacore AB (Uppsala, Sweden) under the trade name BlAcore (hereinafter "the BIAcore instrument"). The BIAcore instrument employs surface plasmon resonance (SPR) to investigate interactions between molecules at the surface of a sensor chip, and includes a processing unit with liquid handling and optical systems, a sensor chip, and a computer for control and data evaluation. One molecule, referred to as the "ligand," is immobilized on the surface of the sensor chip, and the other molecule, the "analyte," flows over the surface of the sensor chip. As the analyte interacts with the immobilized ligand, SPR is used to measure a change in refractive index on the surface of the sensor chip. Selective interactions of the analyte to the immobilized ligand gives this technique specificity, and also enables analysis of interactions in complex mixtures.
The BIAcore instrument has been used extensively, and a large volume of literature has been published concerning its operation and applicability. For example, published PCT WO 90/05295 discloses in greater detail the construction and operation of the BIAcore instrument, while published PCT applications WO 90/05303 and WO 90/05305 are directed to various sensor surfaces for use therewith. Further, the BIAtechnology, BIAapplication, and BIAcore Handbooks published by BIAcore AB describe in considerable detail the operation and construction of the BIAcore instrument.
In general, an analyte present within a liquid sample interacts with the ligand associated with, for example, a dextran matrix bound to the surface of the sensor chip. Binding of the analyte by the ligand gives rise to an increase in refractive index which is monitored in real time by a change in the resonance angle as measured by SPR. The data take the form of a sensorgram which plots the signal in resonance units (RU) as a function of time. A signal of 1,000 RU corresponds to the binding of about 1 ng of analyte per mm.sup.2 (Johnsson et al., Anal. Biochem. 198:268-277, 1991; Fagerstam et al., J. Chromatography 597:397-410, 1992; Stenberg et al., Colloid and Interface Science 143:513-526, 1991).
During operation of the BIAcore instrument, the sample is delivered to the sensor chip utilizing an integrated micro-fluidic cartridge (IFC). The IFC consists of a series of precision-cast channels in a hard silicon polymer plate, forming sample loops and flow channels for buffer and sample delivery. The IFC is pressed into contact with the sensor chip by a docking mechanism within the BIAcore instrument. A representative IFC as used by the BIAcore instrument is depicted in FIG. 1A, which illustrates the channels and valves (as viewed from above), with the inset showing a side view of the same and depicting a flow cell formed from pressing the IFC against the sensor chip.
Sample flow through the IFC is controlled by a set of pneumatically actuated diaphragm valves which direct the sample through the various channels to the sensing surface of the sensor chip. In this manner, the BIAcore instrument (e.g., BIAcore 2000) permits single or multichannel analysis in up to four flow cells. For example, FIG. 1B illustrates sample being passed through three flow cells in series (labeled FC 1, FC 2 and FC 3). Although not specifically depicted in FIG. 1B, sample can also pass through just a single flow cell for analysis (e.g., FC 1).
Existing BLAcore instruments employ flow cells having a cross sectional area of 0.05.times.0.5 mm and a length of 2.4 mm, giving a volume of about 60 nanoliters (nl), and having a total sensing surface area in each flow cell of approximately 1.2 mm.sup.2. A focused incident light illuminates approximately 1.6 mm of the length of the sensing surface for each flow cell, with the detector imaging about 0.17 mm of the width of the sensing surface. This corresponds to a sensing area within each flow cell of about 0.3 mm.sup.2. Each flow cell in the BIAcore instrument contains a single sensing area. Thus, if the sample is to contact four different sensing areas, passage of the sample through four separate flow cells is required (i.e., FC 1, FC 2, FC 3 and FC 4).
While sample delivery to multiple flow cells as presently employed in the BLAcore instrument offers numerous advantages, and represents the state of the art with respect to sample delivery techniques, improvements thereto are still desired. For example, in the context of kinetic measurements, it is important that sample be delivered to each flow cell in a well-defined volume or "plug," with minimal dispersion at sample-buffer boundaries. Such a sample plug is created by switching between sample and buffer flow in the IFC with aid of the pneumatic valves. While dispersion is minimized by keeping dead volumes between the valves and flow cells small, there are still periods at the start and end of sample introduction where the concentration of the sample is diluted by dispersion (i.e., mixing of the sample with the running buffer in the system). Further, dispersion increases with the number of flow cells in series (as depicted in FIG. 1B). Such dispersion results in a time lag in both the rise and fall of the sensorgram at the beginning and end of sample introduction. These so-called "rise and fall times" limit the ability to resolve fast reaction kinetics (i.e., interactions with high rate constants). One way to solve this limitation is to increase the flow rate. Unfortunately, increasing the flow rate means increased sample consumption. There are also practical and design limitations in terms of, for example, liquid pressure which provide an upper limit for the flow rate.
In addition, temperature variations between flow cells can negatively impact sample analysis. Since refractive index, reaction kinetics and mass transport of the analyte to the sensing surface are all sensitive to temperature, it is important that such measurements be carried out at controlled temperatures. Due to physical separation of the flow cells, and hence the sensing surfaces, temperature fluctuations between flow cells can be a source of measurement error. Further, the flow cells depicted in FIGS. 1A and 1B do not permit controlled sample delivery to discrete areas within a single flow cell, nor do they allow immobilization of different ligands to discrete sensing areas within a single flow cell. Rather, such modifications are only achieved within separate flow cells, and thus are accompanied by the limitations as noted above.
Accordingly, there is a need in the art for improved sample delivery techniques within the context of flow cell-based detection instruments, such as the BIAcore instrument, as well as for other instruments of similar design or operation. To that end, any instrument which detects a measurable change in a property associated with a flow cell-based sensing structure may benefit from improved sample delivery techniques. Such improvements should provide fast liquid exchange rates between sample and buffer, maintain constant temperature control across multiple sensing areas, and permit a variety of sample delivery techniques to multiple sensing areas within the flow cell.
The present invention fulfills these needs and provides further related advantages.